The part of the containers that is the most difficult to manufacture is the base. It is effectively the base which supports the weight of the column of liquid contained in the container and, if it has insufficient mechanical strength, it loses its shape and the container can no longer stand in a stable manner on a support.
Now, the base, specifically in order for it to be able to have the desired mechanical strength, has an appreciable wall thickness, that is proportionately greater than that of the rest of the container. This means that, in relative terms, it requires more material than the body of the container the wall of which has currently been reduced down to a very small thickness that is becoming difficult to reduce further. The base is therefore that part of the container in which reductions in thickness, and therefore savings in thermoplastic, can still be envisioned.
What is more, the base is the region of the container that is the most difficult to shape correctly in the blow-molding or stretch-blow-molding operation, specifically because the appreciably greater thickness of material makes it slower to stretch and less easy to shape. Now, to improve its mechanical strength, the base generally has a complex or even very complex shape with numerous recessed and protruding features the correct shaping of which leads to the desired mechanical strength.
These difficulties and disadvantages, which are of a general nature, are further aggravated in containers that have complex shaped bases with multiple projecting feet known as “petalloid” bases. Indeed, these petalloid bases have, around the periphery, an alternation of substantially axial projecting lumps that form the feet and of recesses or “valleys” which separate them and the bottoms of which run radially approximately over a hemispherical envelope: the stretching, particularly in the axial direction, of the material during the blow-molding process is therefore done to extents which vary very widely according to whether the region concerned is in a foot or in a valley. Now, at all points on the base, the mechanical strength has to be high enough to prevent the base from collapsing under the action of the weight of liquid and the internal pressure (this type of container often being used to contain carbonated liquids), this being obtained by providing a substantially increased thickness of material. Furthermore, the complex shape of petalloid bases requests tight control over the blow-molding conditions, particularly the blowing pressure, in order to be able to shape the bases correctly.
Furthermore, manufacturers of thermoplastic containers are constantly seeking to improve the economic production conditions by attempting to produce containers that require a smaller amount of thermoplastic, and at ever-increasing production rates.
It is specifically an object of the invention to meet these requirements of everyday practice and to propose a technical solution that is improved with regard to the manufacture of the base of the containers, which is capable of leading to appreciable savings in raw materials and at the same time to an appreciable increase in production rates.